Conventional reinforcing methods of organic polymer materials may be classified roughly to the followings.
[1] Case that there is No Compatibility Between Materials
1) Improvement by Mixing Fillers (Reinforcing Materials)
This method is used for solidifying by enveloping the fillers with a thermosetting resin. Generally, a reinforced composite having properties of the fillers is produced by charging the thermosetting resin before curing with fine particles of fillers which are inorganic materials or immiscible resins, and then curing the resin. In case that the fillers are simply mixed with the thermosetting resin without crosslinking or curing, however, since the resin matrix is diluted by the fillers to generate exfoliating of the resin from the fillers at their boundary surface, contrary to expectation, the strength is totally lowered in general and reinforcement of physical properties can not be expected. In order to improve the strength, it is essential to enhance the bonding force between both by applying some methods such as addition of bonding agents. In the field of medical materials, use of an unknown binder such as a coupling agent which is necessary to investigate its toxicity is not desired.
In view of the closest packing mechanism, in order to exhibit chemical, physiological properties of fillers sufficiently, it is necessary to fill with the fillers in an amount of more than 33.3. vol % which is an amount that the fillers exist continuously throughout a molded material from back side to front side. In case that a large amount of foreign materials are mixed and any binder is not used, it cannot be avoided that a certain kind of properties is remarkably lowered.
2) Improvement by Fiber Reinforcing
This reinforcing method is most effective and is most generally used. In this case, a matrix resin and fibers to reinforce have different natures and are immiscible to each other. The reason is that, when the fiber with the similar nature is added to a matrix resin which is dissolved in a solvent, or melted in a matrix, or in the form of solution such as monomer or oligomer, the fiber is dissolved in the matrix resin and then is destroyed its fibrous structure. High strength fibers such as carbon, glass, KEVLAR and boron are used for reinforcement, because these are essentially different from the matrix resin and cannot be dissolved therein. In medical devices, though there has been studied reinforcement by bioinert fibers or bioceramics fibers which are highly bio-compatible among those fillers, it is very afraid that there is a danger that such a fine fiber stimulates physically cells or organs. Examples of the bioinert fibers include non-biodegradable and non-bioabsorbable PEEK (polyether-ether-ketone), carbon fiber, and the like. A method where reinforcing by using the non-biodegradable and non-bioabsorbable fibers has not yet been clinically used, because those fiber fragments give undesired physical stimulation which is harmful to a living body.
A guidance of fiber reinforcement resistive to breakdown is not to use a strong fiber, but is to use a matrix resin which is tough and is hard to generate cracks, and to prevent exfoliating at the interface of the fiber and the resin. From this viewpoint, though it is one idea to reinforce by using a compatible fiber having same nature, considering the physical stimulation to a living body, it cannot be recommended to use a composite produced by this method as medical materials.
3) Reinforcement by Crystal Orientation
Conventionally, for reinforcing a crystalline polymer, there is employed a method where crystals are oriented in a certain determined direction in order to exhibit intermolecular force of adjacent polymer chains effectively. There is a special method where a film is reinforced by biaxial drawing, but a usual method is to enhance the intermolecular force by uniaxial drawing. In the uniaxial drawing, since the crystals are oriented in the uniaxially drawn direction, there happens anisotropy of strength due to the crystal orientation in the mechanical direction (MD) and the traverse direction (TD) at a right angle thereto. In a molded material that dislikes the anisotropy of strength, it is demanded a reinforcing method where an eccentric crystal orientation is avoided as possible. The press-forging method (Patent Document 1) invented to a poly (lactic acid) by the present inventor falls under this method, which could avoid significantly the anisotropy according to the conventional method by orienting in the multiaxial directions at certain gradient angles to the center axis along with MD.
The forging reinforcing method of Patent Document 1 is effectively applied basically to a crystalline polymer composed of a crystalline phase and a glass phase at a room temperature and has a glass transition temperature of a room temperature or higher, or a composite thereof and organic fine particles. Typical examples of the polymer composed of a glass phase and a crystalline phase at ordinary temperatures and having a glass transition point: Tg (° C.) of a room temperature or higher, and a melting point Tm (° C.) where the crystalline phase is molten are in the followings. Nylon 6 (Tg: 47° C., Tm: 255° C.), Nylon 66 (Tg: 49° C., Tm: 267° C.), polyethylene terephthalate (Tg: 68° C. and 81° C., Tm: 260° C.), polyvinyl chloride (Tg: 82° C., less crystalline phase, Tm: 180° C.), polystyrene (Tg: 100° C., Tm: 230° C.), polymethyl methacrylate (Tg: 70° C., Thermal deformation temperature: 140° C.), poly (lactic acid) (Tg: 65° C., Tm: 185° C.), and the like. Since these are not melted between Tg and Tm, but are convert to be softened in parallel with temperature elevation, plastic deformation can be carried out between Tg and Tm by the forging treatment if applying a pressure larger than that of the case of melt molding. In addition, since crystallization is progressed gradually between those temperatures, strength of material can be changed by varying crystallinity or varying the orientation of crystals. Of course, strength can be extremely increased by enhancing the orientation. By using this principle smartly, the present inventor has reinforced a poly (lactic acid) (PLLA) or a composite (HA/PLLA) of PLLA and hydroxyapatite (HA) fine particles by subjecting to crystal orientation by forging. These forging-reinforced products are now used clinically as a high strength, bio-active, biodegradable bone fixation and joint devices.
The present inventor has also invented a method where the anisotropic mechanical strength is further improved by subjecting the once forged article to a second forging by changing its mechanical direction (MD) (Patent Document 2).